This invention relates to a wake-up circuit for a CAN receiver, and more specifically to a wake-up circuit for the physical layer of an electronic control unit (ECU) on a CAN.
Modem vehicles have abandoned the utilization of separate wires between the various modules on the vehicle because of the sheer bulk of the number of conductors required and the cost associated therewith. The main motivation for using a CAN bus is noise immunity. It is used widely in automobiles, as well as industrial applications where the environment is harsh and levels of electrical interference are very high. These vehicles use a controller area network (CAN) in accordance with International Standard ISO 11898, entitled xe2x80x9cRoad vehiclexe2x80x94Interchange of digital informationxe2x80x94Controller area network (CAN) for high-speed communicationxe2x80x9d, for example.
A simplified view of a CAN network is shown in FIG. 1 generally as 100. In this simplified diagram, other components, such as terminating resistors, are omitted for simplicity. The CAN bus comprises two wires one labeled CANH and the other CANL, as is well known to those skilled in the art. FIG. 1 illustrates three modules 110, 120 and 130 on the bus. The devices are wired in parallel as illustrated with devices 110 and 120 which have lines 112, 114 and 122, 124, respectively connected to the bus. Device 130 is shown as two separate modules 132, 138 to illustrate how the module, referred to as a ECU in the International Standard, is constructed. The module comprises a physical layer 138 coupled to a processor 132. The physical layer is an analog layer which transmits signals along and receives signals from the CAN bus. Microprocessor 132 communicates with the physical layer 138 via a transmit line 134 and a receive line 136. The physical layer 138 is connected to the bus via lines 140 and 142. The detailed operation of this module is described in the International Standard and need not be described in detail here.
In order to control the power consumption of the network the ECU enters a xe2x80x9csleep modexe2x80x9d in which very little quiescent power is drawn by the module. This can occur at any point in the operation of the vehicle in which that particular module is not needed. It can also occur when the vehicle ignition is turned OFF. Although only illustrated when respect to a single module in FIG. 1, many modules on the CAN may be placed in the xe2x80x9csleepxe2x80x9d mode thus multiplying the quiescent current drain by the number of modules. This is especially important when the vehicle is turned OFF, because this current drain will come from the vehicle""s battery.
While in the sleep mode, the circuit needs to be able to detect the presence of a differential signal on the CAN bus and to wake the circuits out of the xe2x80x9csleepxe2x80x9d mode so that the circuit can respond, if the message is addressed to it. Thus, while the circuit must draw as little current as possible in the xe2x80x9csleepxe2x80x9d mode, it must still be able to detect signals on the CAN bus.
Circuits on a CAN bus must be able to withstand voltages much higher than the signal voltage and have a high common mode signal rejection. Thus, there is a need for a wake-up circuit for a circuit on the CAN bus which can meet these requirements while still requiring a very low current drain in the xe2x80x9csleepxe2x80x9d mode.
It is the general object of the invention to provide a wake-up detector for a ECU on a CAN. This and other objects and features are provided by a wake-up detector for an electronic control unit (ECU) on an controller area network (CAN). A first transistor of a first type conductivity has a gate electrode coupled to an input terminal for a CANH conductor of a CAN bus. A second transistor of a second type conductivity has a gate electrode coupled to an input terminal for a CANL conductor of a CAN bus. A first current mirror circuit is coupled to a source-drain path of the first transistor and has an output coupled to a first constant current source. A second current mirror circuit is coupled to a source-drain path of the second transistor and has an output coupled to a second constant current source. A first detector circuit is coupled to a first junction of the first current mirror circuit and the first constant current source. A second detector circuit is coupled to a second junction of the second current mirror circuit and the second constant current source. An OR gate is coupled to an output of the first and second detectors and generates a wake up signal.
Another aspect of the invention comprises a wake-up detector for a physical layer of the ECU on a controller area network (CAN) having a plurality of electronic control units (ECU) coupled to a bus. A NMOS transistor has a gate electrode coupled to an input terminal for a CANH conductor of a CAN bus. A PMOS transistor has a gate electrode coupled to an input terminal for a CANL conductor of a CAN bus. A first current mirror circuit is coupled to a source-drain path of the NMOS transistor and having an output coupled to a first constant current source, the first constant current source comprising a third current mirror circuit coupled between the first junction and a first voltage source. A second current mirror circuit is coupled to a source-drain path of the PMOS transistor and having an output coupled to a second constant current source, the second constant current source comprising a fourth current mirror circuit coupled between the second junction and a second voltage source. A first Schmitt trigger circuit is coupled to a first junction of the first current mirror circuit and the first constant current source. A second Schmitt trigger circuit is coupled to a second junction of the second current mirror circuit and the second constant current source. An OR gate is coupled to an output of the first and second Schmitt trigger circuits and generates a wake-up signal.
A further aspect of the invention includes a method for detecting a signal on a controller area network (CAN) bus for waking-up electronic control unit (ECU) coupled to the bus. A CANH conductor of the CAN bus is coupled to a first transistor of the CAN bus to a first transistor of the first type. A CANL conductor of the CAN bus to a second transistor of a second type. An output of the first transistor is coupled to a first current mirror circuit and an output of the second transistor to a second current mirror circuit, the current mirror circuits mirroring current in source-drain paths of the of the respective transistors. An output of the first current mirror circuit is compared to a first constant current source. An output of the second current mirror circuit is compared to a second constant current source. A wake-up signal is generated if either comparing step equals or exceeds a predetermined level.